1. Field of the Invention
The present invention relates to an image forming apparatus that forms a visible image on an image bearing member to obtain a recorded image, a belt unit used in the image forming apparatus, and an image forming system including the image forming apparatus.
2. Discussion of the Background
Image forming apparatuses in which a recorded image is obtained by forming a visible image on an image bearing member are known, e.g., an electrophotographic copying machine, a printer, a facsimile apparatus, and a multi-function apparatus having at least two functions of copying, printing and facsimile functions. Various types of image forming apparatuses having different functions are known. For example, some image forming apparatuses can form an image of relative large size, others can only form an image of relatively small size. Further, while some image forming apparatuses can form an image on each side of a recording medium, others form an image only on one side of a recording medium. Each user selects and obtains one of these image forming apparatuses having different functions based on its needs. However, it often happens that after obtaining an image forming apparatus having certain functions, the needs of the user change and it becomes necessary to obtain image forming apparatuses having different functions. In this case, the user is forced to bear a great financial burden.
An image forming apparatus is known to form a full color image on each side of a recording medium by first transferring visible images of different colors formed on a plurality of image bearing members onto an intermediary transfer member by superimposing one image of different color upon another, thereby forming a first full color image thereupon, transferring the first full color image onto an intermediary image bearing member, then transferring further visible images of different colors subsequently formed on the plurality of image bearing members onto an upper surface of a recording medium being conveyed between the plurality of image bearing members and the intermediary transfer member by superimposing one further image of different color upon another, thereby forming a second full color image on the upper surface of the recording medium, and further, transferring the first full color image on the intermediary image bearing member onto a lower surface of the recording medium.
In the above-described image forming apparatus, if an image to be formed is smaller in size than a surface size of the intermediary image bearing member, the image can be formed on each side of the recording medium. Accordingly, if the surface size of the intermediary image bearing member is set to such a size that corresponds to a recording medium of a maximum size that can be used in the image forming apparatus, a color image can be formed on each side of any recording medium used in the image forming apparatus. For example, when the maximum size of a recording medium that can be used in an image forming apparatus is A3, by setting the circumferential length of an intermediary image bearing member of the image forming apparatus to a size greater than the long side of a recording medium of A3 size and the width of the intermediary image bearing member to a size greater than the short side of the recording medium of A3 size, a color image can be formed on each side of any recording medium used in the image forming apparatus. However, providing such an image forming apparatus including an intermediary image bearing member having such a large surface size results in a higher cost apparatus relative to an image forming apparatus including an intermediary image bearing member having a smaller surface size.
Demands of users for an image forming apparatus vary, as described above. Thus, some users only use a recording medium of A4 size as a maximum. If this user can only obtain an expensive image forming apparatus including an intermediary image bearing member having a large surface size corresponding to a recording medium of, e.g., A3 size, the user is forced to bear an unnecessary cost burden in light of their needs. Further, when an intermediary image bearing member having a relatively long circumferential length is used, as compared to using an intermediary image bearing member of shorter circumferential length, the time required for forming an image is increased, resulting in decreased image forming efficiency.
Conversely, a user who has obtained an image forming apparatus including an intermediary image bearing member that is small in surface size because of an existing need to form an image on each side of only a small-sized recording medium has a problem if a need to form images on each side of a larger sized recording medium arises. In this case, the user must obtain a second image forming apparatus that includes an intermediary image bearing member that is of a sufficiently large surface size to meet this new need, and thereby bear an excessive cost burden. The same kind of problem arises when the user who has obtained an image forming apparatus including an intermediary image bearing member having a large circumferential surface no longer needs to form an image on each side of a large-sized recording medium any more and desires to obtain an image forming apparatus including an image bearing member having a small circumferential surface.
Another image forming apparatus is known, in which at least one image bearing member is provided, visible images of different colors are formed on the image bearing member, and the visible images of different colors are transferred onto an intermediary transfer member by superimposing one image of different color upon another so that a full color image is formed on the intermediary transfer member. The full color image is then transferred onto a recording medium by a transfer device. The above-described image forming apparatus forms a color image on only one side of a recording medium. Still another image forming apparatus is known, in which a first image formed on an intermediary transfer member by superimposed visible images transferred from an image bearing member is transferred onto an intermediary image bearing member. The first image is then transferred onto a first side of a recording medium and at the same time a second image formed on the intermediary transfer member by subsequent superimposed visible images transferred from the image bearing member is transferred onto the second side of the recording medium. The image forming apparatus having this configuration forms a color image on each side of a recording medium in a relatively short time.
The former image forming apparatus forms a color image only on one side of a recording medium, whereas the latter image forming apparatus forms a color image on each side of a recording medium. Therefore, in terms of functions, the latter image forming apparatus is superior to the former image forming apparatus. However, the cost of making the latter image forming apparatus is higher than that of the former image forming apparatus, so that the price thereof is also higher than that of the former image forming apparatus.
Also, an image forming apparatus is known, in which visible images are formed on at least one image bearing member and the visible images are immediately transferred by a transfer device onto a recording medium, being superimposed one upon another, thereby forming a color image on the recording medium. Further, another image forming apparatus is known, in which visible images formed on an image bearing member are transferred onto an intermediary image bearing member, being superimposed one upon another, thereby forming a first image thereupon, the first image is then transferred onto one side of a recording medium, and visible images subsequently formed on the image bearing member are transferred onto the other side of the recording medium, being superimposed one upon another, thereby forming a second image on the other side of the recording medium. In this case also, the former image forming apparatus forms a color image only on one side of a recording medium, whereas the latter image forming apparatus forms a color image on each side of a recording medium. Therefore, in terms of functions, the latter image forming apparatus is superior to the former image forming apparatus. However, the cost of making the latter image forming apparatus is higher than that of the former image forming apparatus, so that the price thereof is higher than that of the former image forming apparatus.
As described above, demands of users for an image forming apparatus are diverse because while one user desires to obtain an image forming apparatus capable of forming an image on each side of a recording medium at a high speed even if the price is high, while another user prefers an inexpensive image forming apparatus even if an image cannot be formed on each side of a recording medium. The former user obtains an image forming apparatus capable of forming an image on each side of a recording medium, and the latter user obtains an image forming apparatus capable of forming an image only on one side of a recording medium. However, the latter user who has obtained an image forming apparatus capable of forming an image only on one side of a recording medium may be later required to obtain an image forming apparatus capable of forming an image on each side of a recording medium because of a change in needs. In this case, this user must purchase a second image forming apparatus that is capable of forming an image on each side of a recording medium, being forced to bear a great financial burden as to purchasing two different image forming apparatuses.